1. Field of the Invention
The present invention relates to a motor, and more particularly to a motor adapted to be built in a lens barrel to drive a diaphragm vane or lens.
2. Description of the Prior Art
In a conventional stepping motor, an excitation coil and pole teeth can be constructed in a hollow structure and a magnet rotor can also be constructed in a hollow structure. Accordingly, it is suitable to drive a diaphragm vane or lens in a lens barrel.
A structure to drive the diaphragm vane by a drive force of the stepping motor is disclosed in Japanese Patent Application Laid-open 165825/1982.
When the diaphragm vane or the lens is to be driven, a power souce for the motor may be a battery in the lens barrel, a battery in a camera body or a battery separate from the camera body. None of those can store a large capacity of power supply energy. As a result, if a rotation efficiency between the rotor and the stator is low, the power supply energy is wasted and the product value of the lens with a built-in motor is lowered. If the rotation between the stator and the rotor is not smooth, the diaphragm vane and the lens may not be moved or rotated to a position determined by an exposure control system or an auto-focus system and it adversely affects the lens performance.
In the prior art stepping motor, the stator is fixed to an outer ring of a bearing, the magnet rotor is fixed to an inner ring of the bearing, and balls are arranged between the outer ring and the inner ring. Because of such a structure, the friction coefficient is high and the rotation efficiency is low. This manufacturing step is also time-consuming.